Particle Size

Asphalt emulsion particle size is primarily created by mechanical mill shear, mill pressure and mill gap in the presence of an emulsifier solution. One could make an emulsion with hot asphalt, soap, and a whisk. While this will emulsify, it will also rapidly separate even with sufficient heat and mixing. The primary reason is particle size; a colloid mill produces much smaller particle size than a whisk and some elbow grease, no matter how hard you churn. Stoke’s Law states this concretely; the smaller particle size and the tighter “packing” of particles produces a more stable emulsion.

Once emulsified, asphalt droplets are stabilized by emulsifiers and other materials within the soap solution. Greater emulsifier concentration, if properly and sufficiently activated per manufacturer’s recommendations, will produce both finer and more stable particles of asphalt in the emulsion. Practically speaking, this sustains the stability created by the initial mill shear.

Produced emulsion grades in a manufacturing facility should be assessed for particle size at regular intervals, such as start-up of production season or after mill maintenance is performed, and as part of the QA process when unexpected emulsion production results occur. Larger than normal particle size could result from improperly produced soap, lower than average asphalt temperatures or several other factors. This could cause the emulsion to settle or separate faster than typically expected.

To produce proper particle size in an asphalt emulsion, proper manufacturing SOPs are essential. Soap manufacturing should track order of addition, weights of materials and soap temperatures, as improperly produced soap solutions will produce unexpected particle quality from inactivated emulsifiers or other issues. Similarly, asphalt temperature and viscosity should be tracked from grade to grade as asphalt should reach around 200 cP when pumping towards the mill and should be expected to lose about 15-20 degrees F in the process of reaching the mill. Sufficient back pressure of 10-25 P.S.I. should be used as well, as it will reduce the effective equiviscous temperature (the temp at which the asphalt has a viscosity of 200cP) required for quality particle size in the emulsion. The colder the asphalt, the more back-pressure will be required to allow for successful particle creation during milling, with highly viscous asphalts such as polymer modified or very low penetration asphalt requiring even more pressure and temperature.

By: Brian Cancio (Russell Standard)